1. Field of the Invention
The present invention relates to a novel liquid crystal compound and a liquid crystal composition that are useful as a material for an optical device. More specifically, it relates to a novel liquid crystal compound that has a wide liquid temperature range of a liquid crystal phase, a large dielectric anisotropy, a large refractive index anisotropy and good compatibility with other liquid crystal compounds, is capable of being used in a wide temperature range and of being operated at a low voltage when the compound is used in an optical device, and is capable of providing steep electrooptical characteristics, a liquid crystal composition containing the compound, and an optical device containing the composition. The term “optical device” referred herein includes various devices exerting such functions as optical modulation and optical switching by using an electrooptical effect, and examples thereof include a display device (liquid crystal display device) and an optical modulation device that is used in an optical communication system, an optical information processing system and various sensor systems.
2. Related Art
A display device using a liquid crystal compound has been widely used in displays of a clock, a desktop calculator, a word processor and so forth. The display device is operated by using the refractive index anisotropy, the dielectric anisotropy and so forth of the liquid crystal compound.
Examples of the operation mode of liquid crystal in a liquid crystal display device includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode and so forth, which use at least one polarizing plate for displaying. Examples of the mode using no polarizing plate include a polymer-liquid crystal composite film, a polymer-dispersed liquid crystal (PDLC) (disclosed in J. of Chem. Soc. of Japan, No. 10, 1019 (1992) and SID Symposium Digest Tech Papers, 16, 68 (1985)), a guest-host mode, a polymer stabilized cholesteric liquid crystal (PSChLC) (disclosed in J. of Japanese Liq. Cryst. Soc., vol. 11, No. 2, 137 (2007), Mol. Cryst. Liq. Cryst., 200, 197 (1991), Liq. Cryst., 24, 387 (1998) and Liq. Cryst., 24, 397 (1998)), in which an electric field is applied to a liquid crystal composite material to switch between a transparent state and a scattered state or between a transparent state and a colored state.
The devices are classified based on the driving mode into a passive matrix (PM) mode and an active matrix (AM) mode. The passive matrix (PM) mode is classified into a static mode, a multiplex mode and so forth, and the active matrix (AM) mode is classified into a thin film transistor (TFT) mode, a metal insulator metal (MIM) mode and so forth.
The liquid crystal display device contains a liquid crystal composition having suitable characteristics. For enhancing the characteristics of the liquid crystal display device, a liquid crystal composition that has suitable characteristics is preferably used. General characteristics that are necessary for a liquid crystal compound as a component of the liquid crystal composition are as follows.                (1) The compound is chemically stable and physically stable.        (2) The compound has a high clear point (phase transition temperature from a liquid crystal phase to an isotropic phase).        (3) The compound has a low minimum temperature of a liquid crystal phase (such as a nematic phase, a cholesteric phase, a smectic phase and a liquid crystal phase that is optically isotropic, e.g., a blue phase).        (4) The compound is excellent in compatibility with other liquid crystal compounds.        (5) The compound has a suitable dielectric anisotropy.        (6) The compound has a suitable refractive index anisotropy.        
In the case where a composition containing a liquid crystal compound that is chemically and physically stable as in (1) is used in a display device, a voltage holding ratio can be increased.
A composition containing a liquid crystal compound that has a high clear point as in (2) or a low minimum temperature of a liquid crystal phase as in (3) enhances a temperature range of a nematic phase or an optically isotropic liquid crystal phase, and thus the display device can be used in a wide temperature range.
A liquid crystal compound is generally used as a composition by mixing with other various liquid crystal compounds for obtaining properties that cannot be exhibited with a sole compound. Accordingly, a liquid crystal compound used in a display device preferably has good compatibility with other liquid crystal compounds as in (4).
A liquid crystal display device that is further enhanced in display performance, for example, in contrast, display capacity, response time and so forth is demanded in recent years. Furthermore, a liquid crystal composition that has a low driving voltage is demanded as the liquid crystal material used.
It has been well known that a threshold voltage (Vth) in a nematic liquid crystal is shown by expression (a) (H. J. Deuling, et al., Mol. Cryst. Liq. Cryst., 27, 81 (1975)):Vth=π(K/∈0Δ∈)1/2  (a)
In expression (a), K represents an elastic constant, and ∈0 represents the dielectric constant of vacuum. As being understood from the expression, there may be two ways for decreasing Vth, i.e., the value of Δ∈ (dielectric anisotropy) is increased, or K is decreased. It is the current situation that, in general, a liquid crystal material having large Δ∈ is used for fulfilling the demand. Under the circumstances, there have been earnest developments of a liquid crystal compound having a suitable dielectric anisotropy as in (5), particularly a liquid crystal compound having a large dielectric anisotropy.
For attaining favorable display with nematic liquid crystal, it is preferred that the thickness of the cell constituting the liquid crystal display device and the value of Δn (refractive index anisotropy) of the liquid crystal material used are constant (E. Jakeman, et al., Phys. Lett., 39A, 69 (1972)). The response speed of the liquid crystal display device is inversely proportional to the square of the thickness of the cell used. Accordingly, for producing a liquid crystal display device having a high response speed capable of being applied to display of motion images, it is necessary to use a liquid crystal composition having a large refractive index anisotropy. Consequently, a liquid crystal compound having a suitable refractive index anisotropy as in (6), particularly a liquid crystal compound having a large refractive index anisotropy, is demanded.
As a liquid crystal compound having a halogen-replaced naphthalene ring, the following compounds having a fluoronaphthalene ring have been known (disclosed in JP 2000-63305 A/2000)

Compounds having a chloronaphthalene ring are disclosed as formulas in WO 2000-17287 A, JP 2000-119653 A/2000, JP 2001-11460 A/2001, JP 2001-11454 A/2001 and JP 2001-26779 A/2001.
In the liquid crystal used in the mode that does not require a polarizing plate, it is preferred to use a liquid crystal compound having a large dielectric anisotropy for driving a device at a low voltage, and the use of a liquid crystal having a large refractive index anisotropy can increase the scattering intensity and enhance the wavelength band of the selective reflection wavelength.
A polymer-dispersed liquid crystal using a liquid crystal composition containing a liquid crystal compound having a fluoronaphthalene ring has been known (disclosed in JP 2005-232214 A/2005, JP 2005-232215 A/2005, JP 2005-250223 A/2005 and JP 2005-257274 A/2005).